Monolithic ZrB2 Research Articles

Contour maps of mechanical properties in ternary ZrB2-TiC-SiC ceramic system

Monolithic ZrB2, ZrB2-SiC, ZrB2-TiC and ZrB2-SiC-TiC were fabricated by hot pressing. Contour maps depicting grain size and mechanical properties were generated on the ZrB2-SiC-TiC ternary diagram through experimental data and software fitting. These visual representations effectively establish the relationships between chemical composition and material properties, despite the similar high hardness of three main compounds. Notably, the average grain size of ZrB2-SiC-TiC is significantly reduced by an order of magnitude compared to monolithic ZrB2. Both strength and hardness exhibit a strong correlation with grain size, while the elastic modulus demonstrates insensitivity to microstructure changes. The evolution of toughness proves to be complex.

Effects of thickness ratio on phase structures, mechanical properties, and wear behaviors of CrN/ZrB2 bilayer films

CrN/ZrB2 bilayer films leveraging the wear resistance of CrN and the mechanical properties of ZrB2 were successfully produced and characterized. DC magnetron sputtering was used to deposit CrN/ZrB2 bilayer films with a thickness of approximately 1 μm and various ZrB2/CrN sublayer thickness ratios (TZrB2/TCrN = 10.85, 5.78, 4.50, 2.29, and 1.04). The thickness ratio was controlled by adjusting the deposition times of the individual sublayers. The effects of thickness ratio on the films' phase structures, mechanical properties, and wear resistance were investigated. The ZrB2 sublayer initially formed with a nanocrystalline structure; as its thickness increased, its texture became (001). As the thickness of the CrN sublayer increased, the CrN (111) reflection became more dominant, and the hardness of the CrN/ZrB2 bilayer films decreased from 19.8 to 11.6 GPa. In the CrN/ZrB2 bilayer films, the CrN sublayer was softer than the harder ZrB2 sublayer; specifically, the wear rate of the monolithic ZrB2 film was 6.45 × 10−5 mm3/N·m. This hardness enabled the bilayer films to exhibit excellent wear resistance with wear rates of 1.04–1.68 × 10−6 mm3/N·m. The wear behaviors of the CrN/ZrB2 bilayer films were classified into two tribological mechanisms in accordance with wear track observations. For the samples with high TZrB2/TCrN ratios, the shear mechanism dominated the wear behavior and resulted in the formation of a transfer layer on the worn surface. By contrast, the samples with low TZrB2/TCrN ratios had a ploughing mechanism in which a deformed track was generated on the surface.

Effect of TaC and graphene nanoplatelets on the microstructure and mechanical properties of ZrB2

In this experiment, in order to investigate the effect of TaC addition on the microstructure, densification and mechanical properties of ZrB2 composites reinforced with graphene nanoplatelets (GNP), three composites were fabricated via spark plasma sintering (SPS) at 1900 °C. Relative density was determined by Archimedes method. The XRD and EDS analyses were used to explore the phase composition and elemental distribution. Also, Field emission scanning electron microscopy (FE-SEM) was employed to characterize the microstructure (grain size and porosity). Flexural strength, fracture toughness and hardness were determined by three-point bending test, single-edge notched beam (SENB) and macro and micro Vickers methods, respectively. The results showed that the presence of the GNP, alone and combined with TaC, increases the relative density sharply to 99.6 %. In addition, full densification occurred by the addition of 10 and 20 vol% TaC compared to the monolithic ZrB2 ceramic with 83 % relative density. Maximum flexural strength and fracture toughness of 580 MPa and 4.5 MPa.m0.5 were obtained, respectively, for ZrB2- GNP and ZrB2- GNP doped with 10 vol% TaC.

Sinterability and flexural strength of monolithic ZrB2: role of nano-carbon black and SiC

Sinterability and flexural strength of monolithic ZrB2: role of nano-carbon black and SiC

Synthesis of zirconium boride (ZrB2) rod crystals through salt-assisted boro/carbothermal reduction

High-aspect-ratio ZrB2 rod crystals were synthesized via a molten-salt-mediated boro/carbothermal reduction using ZrOCl2· 8H2O/B4C/C as the raw material and NaCl as the flux material. This synthesis may improve the poor fracture toughness of monolithic ZrB2 ceramics. X-ray diffraction and scanning electron microscopy confirmed that the heat-treatment temperature of 1550 °C and the addition of 50 wt% NaCl were the most effective in the preparation of ZrB2 grains with a high aspect ratio. Thermogravimetric and differential scanning calorimetry was used to study the influence of NaCl addition, which increased the liquid-phase content. Finally, the growth of ZrB2 grains along the c-axis into rod-like morphology at elevated temperatures was investigated by transmission electron microscopy. This work revealed that the addition of NaCl has a positive effect on the reducing of synthesis temperature and increasing of products particle size. The ZrB2 rod crystals synthesized have diameters and aspect ratios of approximately 1 μm and 30, respectively.

Thermal conductivity of hot-pressed ZrB2-ZrC ceramics

The thermal conductivity of ZrB2 with 0–10 vol.% ZrC was studied from 25 to 2000 °C. The ZrB2-ZrC ceramics were prepared by attrition milling the starting powders and adding 0.5 wt.% C as a sintering aid. The ceramics were hot-pressed to full density at 1900 °C for 30 min in a flowing Ar atmosphere. The ceramics contained graphitic carbon as a secondary phase, with ZrB2 and ZrC grain sizes of approx. 3.5 and 1.5 μm, respectively. At room temperature, thermal conductivity was 75 W/m·K for monolithic ZrB2, increasing to 79 W/m·K for 2.5 vol.% ZrC, and decreasing to 72 W/m·K for 10 vol.% ZrC. At 2000 °C, thermal conductivity was 72 W/m·K for ZrB2, decreasing to 69 W/m·K for 10 vol.% ZrC. Electrical resistivity behavior was similar to thermal conductivity, decreasing with the addition of ZrC up to 2.5 vol.% (9.9 µΩ·cm at 25 °C), then increasing with further ZrC additions. The increase in thermal conductivity with ZrC addition up to 2.5 vol.% was the result of preferential segregation of W from the ZrB2 phase to the ZrC phase. The reduction in thermal conductivity with greater ZrC additions was due to the lower thermal conductivity of ZrC.

Development of functionally graded ZrB2–B4C composites for lightweight ultrahigh-temperature aerospace applications

In the present work, a lightweight three-layer ZrB2–B4C functionally graded composite material has been developed by spark plasma sintering route. The functionally graded material (FGM) is free from interlayer defects and displays a smooth transition between the individual layers. The composition of each layer was designed to reduce the overall density without sacrificing the functionality of the material. The density of the FGM is almost 40% lower than monolithic ZrB2 and 24% lower than ZrB2–30B4C rendering it potentially very attractive for high-temperature aerospace applications. A detailed structural characterization of the FGM was carried out to evaluate the elemental distribution in the graded composite, as well as determine the spatial distribution of the crystalline phases. Vickers hardness was measured within each layer and in the interlayer regions to further evaluate the gradient structure and interlayer transitions. Longitudinal elastic constants of the FGM along the thickness and across the layers measured using ultrasound phase spectroscopy showed that despite the gradient structure, the FGM can be treated as a quasi-isotropic solid.

Dry sliding wear behaviour of ZrB2-based ceramics: Self-mated and cross coupling with alumina

Systematic dry sliding wear tests with monolithic ZrB2 and Al2O3 pins coupled to ZrB2, ZrB2-20 vol% SiC and Al2O3 discs were carried out in a disc-on-pin configuration. The steady state friction of ZrB2 self-mated or cross coupled with Al2O3 was about 1.1. Self-mated monolithic ZrB2 discs worn about three orders of magnitude more than self-mated Al2O3 discs. ZrB2 pin wear rate was almost double when coupled to ZrB2 or ZrB2-20 vol% SiC discs than when coupled to Al2O3 discs. The wear track of ZrB2-based materials showed an oxygen increment due to humidity-driven tribo-reaction. In all the systems, the main wear mechanisms observed were microfracture and abrasion. Numerical calculations and fracture models were employed to describe the wear mechanisms. By nanoindentation tests on worn and unworn areas, a significant lower hardness of the debris layer formed when ZrB2 materials were involved.

Combined and alone addition effect of nano carbon black and SiC on the densification and fracture toughness of SPS-sintered ZrB2

ZrB2-nano-size carbon black (C.Bn), ZrB2-SiC, and ZrB2-SiC-C.Bn specimens were fabricated by spark plasma sintering (SPS) at 1850 °C for 8 min under 35 MPa pressure. The bulk density (B.D) was measured by the Archimedes method. The single-edge notched beam (SENB) method was applied to investigate the fracture toughness and work of fracture (WOF). It was found that the combined addition of C.Bn and SiC modified the densification behavior to reach full-densified samples. Also, it was found that the fracture toughness and WOF were improved by the addition of C.Bn to the monolithic ZrB2 continuously, and they reached 5.1 ± 0.1 MPa·m1/2 and 433 ± 20 J·m−2, respectively for the sample containing 15 vol% C.Bn. Compared to monolithic ZrB2, the fracture toughness and the WOF for the composite with 30 vol% SiC and 15 vol% C.Bn increased nearly 26 % and 57 %, respectively. It was noteworthy that the alone addition of C.Bn to the pure ZrB2 had more effect in improving the fracture toughness than the simultaneous addition of C.Bn and 30 vol% SiC. The predominant mechanisms in fracture toughness improvement can be related to the observed crack deflections, branching, and bridging in scanning electron microscopy (SEM) images.

Fabrication of (Zr,Ti)B2–ZrN–BN composites through reactive spark plasma sintering of ZrB2 and TiN

Mechanical properties and sintering behavior of additive-free and TiN-doped ZrB2 ceramics were studied. Reactive spark plasma sintering method was applied for manufacturing of ceramics at 1850 °C for 6 min under 40 MPa. The impact of TiN addition on the microstructure evaluation, densification, and mechanical feathers was investigated. A porous monolithic ZrB2 with a relative density of 76.5 % was manufactured, while the introduction of 5 wt% TiN resulted in enhancement of relative density to 93.1 %. The formation of (Zr,Ti)B2 solid solution and in-situ h-BN and ZrN phases was proven by microstructural assessments and X-ray diffractometry. Minimizing the grain growth and improving the densification, as the results of TiN addition, led to enhancement in mechanical properties. The values of bending strength, fracture toughness, and Vickers hardness boosted from 187.6 MPa, 1.9 MPa.m½, and 10.1 GPa for additive-free ZrB2 to 606.5 MPa, 4.5 MPa.m½, and 18.8 GPa for (Zr,Ti)B2–ZrN–BN composite.

Synthesis and Sintering of Nanostructured ZrB2-Al2O3 Composite

ZrB2 is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, ZrB2 applications are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites, and to fabricate nanostructured materials. In this study a dense nanostructured ZrB2-Al2O3 composite was rapidly sintered using the pulsed current activated heating (PCAH) method within 3 min in one step, from mechanically synthesized powders of ZrB2 and Al2O3. Consolidation was accomplished using an effective combination of current and mechanical pressure. A highly dense ZrB2- Al2O3 composite with a relative density of up to 97.4% was fabricated using the simultaneous application of 70 MPa pressure and a pulsed current. The fracture toughness and hardness of the ZrB2-Al2O3 composite were 3.9 MPa.m1/2 and 1917 kg/mm2, respectively. The fracture toughness of the composite was higher than that of monolithic ZrB2.

Synthesis and Sintering of Nanostructured ZrB2-Al2O3 Composite

ZrB2 is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, ZrB2 applications are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites, and to fabricate nanostructured materials. In this study a dense nanostructured ZrB2-Al2O3 composite was rapidly sintered using the pulsed current activated heating (PCAH) method within 3 min in one step, from mechanically synthesized powders of ZrB2 and Al2O3. Consolidation was accomplished using an effective combination of current and mechanical pressure. A highly dense ZrB2- Al2O3 composite with a relative density of up to 97.4% was fabricated using the simultaneous application of 70 MPa pressure and a pulsed current. The fracture toughness and hardness of the ZrB2-Al2O3 composite were 3.9 MPa.m1/2 and 1917 kg/mm2, respectively. The fracture toughness of the composite was higher than that of monolithic ZrB2.

Monolithic ZrB2‐based UHTCs using polymer‐derived Si(Zr,B)CN as sintering aid

AbstractIn the present work, dense ZrB2‐based monoliths were produced via hot‐pressing of ZrB2 powders coated with amorphous polymer‐derived SiCN, SiZrCN, or SiZrBCN at relatively moderate temperatures (i.e., 1800℃). Thus, ZrB2 particles were embedded in polymer‐derived ceramic shell were realized by coating their surface with the corresponding polymeric precursors (polysilazane, Zr‐modified polysilazane, Zr‐ and B‐modified polysilazane, as for SiCN, SiZrCN, and SiZrBCN; respectively), followed by cross‐linking at 200℃ and pyrolysis at 1100℃ in Ar atmosphere. The obtained monolithic samples were carefully characterized concerning their phase composition and microstructure. Typically, the densified samples exhibit a homogeneous microstructure consisting of ZrB2 grains and a multi‐phasic polymer‐derived ceramic grain boundary phase (β‐SiC, t‐BCN, ZrO2, and ZrC). The prepared monoliths exhibited fair hardness values, that is, 12.5, 14.2, and 13.3 GPa for ZrB2/SiCN, ZrB2/SiZrCN, and ZrB2/SiZrBCN.

Oxidation kinetics of ZrB2-and HfB2-powders and their SiC reinforced composites

Non-isothermal simultaneous thermal analysis technique elicited the peak oxidation and mass gain shifts to higher temperatures with SiC addition prompting enhanced activation energy (323 kJ/mol to 593 kJ/mol in ZrB2 and 199 kJ/mol to 207 kJ/mol in HfB2). On one hand where the peak oxidation temperature in ZrB2-SiC pellet could not be observed affirming its high oxidation resistance with negligible mass gain, whereas on the other hand the activation energy of HfB2-SiC pellet increased to 266 kJ/mol. HfB2-SiC powder is observed to follow 3D diffusion dominated (Jander equation) mechanism in which mass gain increased linearly from peak oxidation temperature ∼710−730 °C at 10–20 °C/min heating rates till the maximum programmed temperature of 1200 °C. Though mass gain starts and almost ends within a range of 5 °C at any heating rate in the case of monolithic ZrB2 and HfB2 powders and in ZrB2-SiC composite powder at peak oxidation temperature, it (mass gain) remains almost negligible in spark plasma sintered ZrB2-SiC and HfB2-SiC composites.

Engineered Role of SiC Particle Size on Multi‐Length‐Scale Wear Damage of Spark Plasma Sintered Zirconium Diboride

The engineered role of coarse‐SiC (D50 ≈72 μm) and fine‐SiC (D50 ≈3 μm) particles on the wear damage of zirconium boride (ZrB2) is assessed at different length scales. Monolithic ZrB2 and 20 vol% SiC reinforced ZrB2 with SiC of two different particle sizes are consolidated via spark plasma sintering. Wear tests conducted on sintered samples for examining multi‐length scale performance include fretting (micro‐wear), micro‐scratching (meso‐wear) and ball‐on‐disk (macro‐wear), under dry and unlubricated conditions. The damage assessment and wear mechanism, characterised via scanning electron microscopy and optical profilometry, show Hertzian tensile cracking in all the samples at loads ≥5 N during micro‐scratching. Wear volume, wear rate and coefficient of friction (cof) decreased with SiC reinforcement during macro‐wear. Fine‐SiC reinforced ZrB2 (ZSF) composite showed least cracking at loads ≥5 N owing to its highest fracture toughness (4.6 MPa m1/2) as compared to coarse‐SiC reinforced composite (ZSC, 2.1 MPa m1/2) and monolithic ZrB2 (3.9 MPa m1/2). Hardness of composites increases as compared to ZrB2, and ZSF shows enhanced elastic modulus and fracture toughness. In summary, superiority of ZSF in terms of improved mechanical performance and wear resistance makes it a potential high‐end material for aerospace applications, where particles with high surface impact may cause material deformation and removal.

Thermo-mechanical simulation of ultrahigh temperature ceramic composites as alternative materials for gas turbine stator blades

The efficiency of the Bryton cycle strongly depends on the maximum temperature of the cycle. However, restrictions on metallurgical problems deprive engineers from the benefit of high temperatures. Ultrahigh temperature ceramics can be considered in such cases, instead of traditional materials like M152 superalloy. In this study, SiC reinforced HfB2 and ZrB2 ultrahigh temperature ceramics were proposed as gas turbine stator blades. The heat transfer and stress-strain equations were solved numerically by the finite element method to obtain temperature and stress distributions. The results showed that the maximum thermal stress occurs in vicinity of the cooling ducts where the temperature gradient is maximum. The maximum displacements of 1.2 mm (for HfB2–SiC) and 1.14 mm (for ZrB2–SiC) occur in the upper wall. It can be noticed that the ZrB2–SiC made blade showed lower maximum stress and displacement than those for the HfB2–SiC made one, as a result of lower expansion coefficient of ZrB2–SiC system. The addition of SiC to monolithic HfB2 and ZrB2 ceramics decreases their thermal conductivity and following that, the temperature uniformity in blades reduces. Although the thermal stresses and the probability of failure in these stator blades enhance, the ZrB2–SiC material presented the best performance among the other investigated samples. Both Coulomb-Mohr and Von Mises failure analyses were employed. It was understood that both blades made of HfB2–SiC and ZrB2–SiC composites simply withstand the applied stresses with the safety factors of about 1.5.

Mechanical properties and oxidation behavior of spark plasma sintered (Zr,Ti)B2 ceramics with graphene nanoplatelets

This research focuses on the production and characterization of (Zr,Ti)B2 solid solution systems. The purpose of this study is to understand how solid solution formation influences the structure, mechanical properties, and oxidation resistance of ZrB2 and TiB2. For this, herein, monolithic ZrB2, TiB2, and (ZrxTi1-x)B2 solid solution systems (x = 0.25, 0.50, 0.58, 0.75) were prepared. Moreover, 1 vol% graphene nanoplatelets (GNPs) were added to two different solid solution systems (Zr0.25Ti0.75B2 and Zr0.58Ti0.42B2) without affecting the complete solid solubility of these systems. All powder mixtures were densified by spark plasma sintering at 1800 °C, under 50 MPa with 5 min holding time. The densification, oxidation behavior, microstructure, phases, and mechanical properties (Vickers hardness and indentation fracture toughness) of these samples were systematically investigated. The formation of solid solutions caused an increase in the Vickers hardness and fracture toughness of the samples. Further improvement in mechanical properties and better oxidation resistance were achieved with the addition of GNPs to the solid solution systems.

Enhanced mechanical properties of laminated ZrB2–SiC ceramics with porous Si3N4 interface

In this study, bionic laminated ZrB2–SiC ceramics (LZNw) were fabricated using Si3N4 whiskers as an interface layer. The room-temperature flexural strength and fracture toughness of LZNw were determined to be 528 MPa and 15.3 MPa m1/2, respectively. With increasing testing temperature up to 1500 °C, the flexural strength of LZNw remained higher than that of monolithic ZrB2–SiC ceramics (ZS). Specifically, at the peak temperature of 1500 °C, the flexural strength of LZNw was as high as 380 MPa, while that of ZS dropped to 219 MPa. This outstanding high-temperature strength of LZNw can be ascribed to the porous structure of the Si3N4 whisker interface layer, which absorbs the glassy phase accumulated at the crack tip, and the stepped fracturing pattern, which hinders the flow of the glassy phase, thus avoiding the grain boundary sliding caused by the enrichment of the glassy phase.

ZrB2 matrix composites with Cr3+/Nd3+ doped Al2O3 luminescent nanoparticles for non-contact temperature sensing

The effect of the addition of Cr3+/Nd3+ doped Al2O3 nanoparticles (Al2O3 co-doped with 1% Cr3+ and 1% Nd3+ ions) on the mechanical and luminescent properties of ZrB2 ceramic composites was investigated. ZrB2 based composites with addition of 8, 16 and 32wt.% of alumina nanoparticles were prepared by spark plasma sintering (SPS). Microstructure characteristics were studied by scanning electron microscopy (SEM), elastic modulus by ultrasonic wave transition method, hardness and fracture toughness by the Vickers indentation methods. The addition of alumina nanoparticles had positive effect on the mechanical properties of the composites. Thus, the hardness increased from 14.5 to 16.2GPa and the fracture toughness from 3.91 to 5.89MPa•m1/2 in comparison to the values of the monolithic ZrB2 system. The composite with the highest additive content showed evident luminescent character and the increase of the temperature caused a reduction of luminescent intensity. This is the first report which shows luminescent properties of ZrB2 composites. Moreover, for the first time, luminescent properties of Cr3+ ions were used to develop ZrB2 matrix composites with Cr3+/Nd3+ doped Al2O3 particles with the self-temperature readout functionality via luminescent thermometry.

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB2)-matrix composites were studied. In this way, a monolithic ZrB2 ceramic (as the baseline) and three ZrB2 matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB2 specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB2 matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiCp) and whiskers (SiCw) showed a synergistic effect on the enhancement of mechanical performance of ZrB2-based composites.